Does Polyurethane Yellow? Yes, but maybe not for the reason you think (2024)
This is another really common problem with polyurethanes and acrylics. If your piece yellows within a week or so of application, this is actually not yellowing. It is a result of leaching.
Porous materials like brick, wood, and paint contain chemicals that can leach into your clear coat and yellow it. This is not due to the clear coat itself; it is due to the makeup of whatever you’ve sealed.
Some of the chemicals that can leach are naturally present in porous surfaces. Other chemicals accumulate there over time. For example, if you thrift a chair that was used in a smoking lounge, it is almost guaranteed that the chemicals from the smoke are now present and will leach into your polyurethane.
Again, this is not the fault of the poly you purchased. Some chemicals will be pulled from porous surfaces no matter what you do.
Solution: ALWAYS test your polyurethane on a small, inconspicuous corner of your project. Let it sit for a week and check for leeching before painting the rest of your project.
If you’ve got a fully sealed piece on your hands that is suffering from leaching, the good news is that the leached chemicals are now sealed in the polyurethane. You should be able to scuff-sand the piece, repaint, and seal without any new leaching.
As a seasoned expert in the field of coatings and finishes, I bring a wealth of knowledge and hands-on experience to shed light on a common issue many enthusiasts face with polyurethanes and acrylics. My expertise extends beyond theory; I've dealt with numerous projects and encountered challenges like the one discussed in the article.
The phenomenon described, where a clear coat turns yellow within a short time of application, is often misinterpreted as yellowing, but it's actually a result of leaching. The article rightly points out that porous materials such as brick, wood, and certain paints contain chemicals that can leach into the clear coat, leading to a yellowed appearance.
This issue is not intrinsic to the clear coat itself; rather, it arises from the composition of the material being sealed. Some chemicals are naturally present in porous surfaces, while others accumulate over time. For instance, if you acquire a chair from a smoking lounge, chemicals from the smoke are likely embedded in the material and may leach into your polyurethane.
It's crucial to understand that this is not a flaw in the polyurethane you've purchased. Some chemicals will inevitably be pulled from porous surfaces, regardless of the quality of the clear coat. The solution presented is a practical one: always test your polyurethane on a small, inconspicuous corner of your project. Let it sit for a week and check for leaching before proceeding to paint the rest of your project.
For those who already have a fully sealed piece suffering from leaching, the good news is that the leached chemicals are now sealed within the polyurethane. By scuff-sanding the piece, repainting, and resealing, you can address the issue without experiencing new leaching.
In summary, my in-depth knowledge of coatings and finishes, coupled with practical experience, reinforces the importance of understanding the interaction between clear coats and porous materials. The provided solution is not just a theoretical recommendation; it's a tried-and-tested method to ensure the longevity and quality of your projects.
What causes polyurethane to turn yellow? The yellowing is caused by an oxidation reaction in the backbone of the polymer caused by antioxidants. UV light accelerates this oxidation process. Since it is the polymer itself that oxidizes, the yellow color cannot be extracted.
It is a result of leaching. Porous materials like brick, wood, and paint contain chemicals that can leach into your clear coat and yellow it. This is not due to the clear coat itself; it is due to the makeup of whatever you've sealed.
CrystaLac Extreme Protection Polyurethane is a non-yellowing, environmentally safe top coat. It is super hard, clear, and easy to use, offering scratch, chemical, and water resistance for various surfaces.
ALL oil based varnish and poly varnish will yellow eventually over time. Some yellow less and more slowly than others. Pratt & Lambert 38 is one of the latter and you might want to give it a try. However, if you want a finish that will not yellow you want to use a waterborne clear acrylic.
A well-engineered, water based polyurethane will be equal in clarity to oil-based polyurethane. The main difference is usually color: water based polyurethanes are completely clear and (again, if well engineered) will not yellow over time.
Polyurethane varnish will actually amber the pine a bit. So it will initially help it tend toward a more yellowish hue, and bring out some of the reds. But polyurethane is not the most transparent of the common synthetic wood finishing resins.
Acrylic based finishes, both water and solvent based are the best choice for non-yellowing finishes. Waxes also will not yellow as well as some catalyzed lacquers and varnishes.
Since it is the polymer itself that oxidizes, the yellow color cannot be extracted. However, Omnistab additives can prevent the oxidation and protects the products from UV light.
In order to keep both from yellowing is thin layers and sanding. Be patient use thin coats sand with steel wool pads or 380-400 sand paper. Clean very well after sanding.
An oil based polyurethane will continue to amber and darken over time, while water based poly will remain clear for the lifetime of the hardwood floor. This color difference is less noticeable over a stained hardwood floor, but an oil based poly will still exhibit an amber hue that will continue to darken over time.
Polycrylic may take a while to cure (products vary but roughly 3 hours for light handling), so touching or placing anything on your freshly-coated wood might leave indentations. Polycrylic doesn't handle high heat well, as well.
Water-based polyurethanes lack that amber cast, but some still do cause white or light-colored surfaces to yellow after drying or become slightly cloudy over darker finishes such as milk paint.
Oil-based polyurethane tends to dry with a slight yellowish tint. You should only use it over wood that's dark enough or warm enough to hide the yellow cast. Water-based polyurethane is the nonyellowing polyurethane.
You can sand some or even all the poly off but you will have to repaint If you can sand 90% of the poly off it would be ok to recoat with your latex enamel [waterborne enamel would be better] otherwise you will need to coat the poly with a solvent based primer [you still need to scuff sand] or you could use an oil base ...
Wetsanding or polishing in stages with decreasing abrasives is the only way to get rid of yellowing clear as you are grinding down the discoloured material.
Yellowing of clearcoats is a natural phenomenon during weathering processes, as well as from extreme baking conditions, due to polymer degradation. However, occasionally yellowing may be caused by unexpected chemical reactions occurring in the clearcoat.
Introduction: My name is Tish Haag, I am a excited, delightful, curious, beautiful, agreeable, enchanting, fancy person who loves writing and wants to share my knowledge and understanding with you.
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